Meta-analysis reveals weak but pervasive plasticity in insect thermal limits

Extreme temperature events are increasing in frequency and intensity due to climate change. Such events threaten insects, including pollinators, pests and disease vectors. Insect critical thermal limits can be enhanced through acclimation, yet evidence that plasticity aids survival at extreme temperatures is limited. Here, using meta-analyses across 1374 effect sizes, 74 studies and 102 species, we show that thermal limit plasticity is pervasive but generally weak: per 1 °C rise in acclimation temperature, critical thermal maximum increases by 0.09 °C; and per 1 °C decline, critical thermal minimum decreases by 0.15 °C. Moreover, small but significant publication bias suggests that the magnitude of plasticity is marginally overestimated. We find juvenile insects are more plastic than adults, highlighting that physiological responses of insects vary through ontogeny. Overall, we show critical thermal limit plasticity is likely of limited benefit to insects during extreme climatic events, yet we need more studies in under-represented taxa and geographic regions.

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The data used in this study are available on the OSF database under accession code: https://osf.io/cbhv4/. We provide raw and processed data. We also provide these data in the Supplementary Files as Supplementary Data 1 We undertake a systematic meta-analysis of experimental studies on the plasticity of insects' upper and lower critical thermal limits, including taxon-specific moderators to investigate variation in plasticity.
Data represent global studies that measure insect critical thermal limit plasticity. Peer-reviewed studies were selected for data extraction if they described dynamic tolerance assays where CTmax or CTmin was measured by ramping the temperature until a specified end point. We chose to only evaluate dynamic studies as it was a common metric used in thermal tolerance assays, removed additional sources of methodological heterogeneity, and was a metric already synthesised in other meta-analyses. We required studies to have at least two temperature treatments (including studies where a single treatment was compared to a control), perform a temperature acclimation treatment (all durations of acclimation, including acute hardening and longer-term chronic acclimation, were included and fluctuating temperatures were allowed), and be undertaken in a laboratory. Studies were not included if any variables in addition to temperature were modified (excluding named moderators). Studies were also excluded if the endpoint was recorded for a proportion of the insects assayed only (e.g. CTmax80). A reference list for studies used in the analysis are provided in the Supplementary Information.
Each step was reported according to the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines. Sample size was not predetermined as data were extracted from all studies that met our search criteria. Sample size was checked for all groups and analyses were not run if sample sizes were too small.
In total, Web of Science and Scopus databases found 12,139 unique results. Study abstracts were scanned manually for suitability by a single observer (HW) and selected studies were further examined by their methodology. Data were extracted (arithmetic mean, standard deviation (SD), sample size (N)) from 60 and 52 articles, comprising 92 and 74 species, for CTmax and CTmin respectively. Data were extracted from Figures, Tables and text or the author was contacted to obtain data. Data were recorded in Excel. Moderators were extracted either from the study itself or from published studies and meta-analyses. References for studies from which data were extracted can be found in the Supplementary Information in Supplementary Data 2-4.
Searches were performed in Web of Science and Scopus between July and November 2020 by HW. Searches included globally published studies but were limited to those published in English and were biased towards Europe and North America. The search only included studies published between January 1990 and November 2020. Papers were excluded if they were published before 1990 as Web of Science only had 3 hits pre 1990 so if included, may have introduced bias between the databases. Additionally, methodologies from earlier papers differed drastically eg measuring % individuals falling from a column, which was not easily comparable to other data collected.
Four studies (cited in the Supplementary Data 3) where a very large number of insects were measured were removed from the CTmax dataset as the unusually large sample sizes (n > 700) grossly inflated the study weight and it was deemed this number of insects could not be accurately assessed in one critical thermal limit assay.
Each step was reported according to the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines. Methodology of data collection and analysis are presented to enhance reproducibility the data. Code and data have been made available so that the study can be reproduced on OSF at: https://osf.io/cbhv4/ Not relevant to a meta-analysis study.
Blinding was not relevant to this study. Data were included or excluded by one author (HW) based on a list of criteria.